The present invention relates to a machine tool for engaging and selectively rotating a tool element and for supporting a workpiece in a very accurately predetermined position relative to the tool for machining. The machine tool may move either or both the tool element and workpiece during machining to accomplish a variety of machining operations, such as facing, milling, boring, turning, drilling and reaming.
Machine tools are known wherein the supporting structure for the spindle comprises a vertical column having vertical rails or ways wherein the spindle head is mounted within the ways and guided for vertical movement. The base of the machine supports a carriage or slide on longitudinal waves for rectilinear movement toward and away from the column, and a rotary index table is supported on the carriage for rotary movement about a vertical axis. Depending on the complexity of the machine, the spindle head assembly can include ways for permitting lateral sliding of the spindle in a horizontal direction relative to the column assembly.
In a vertical turning lathe, the tool is held in the spindle against movement, and the table, which has the workpiece clamped thereto, rotates relative to the tool, which is moved into the workpiece to perform a turning operation. The spindle can also be driven, in which case a rotary machining tool is utilized in order to effect facing, milling, drilling, tapping and reaming operations. In some operations, such as milling, the work is rotated by the table into the rotating tool.
The spindle can be oriented either vertically, as in the case of vertical turning lathe, or horizontally, in which case the tool is rotated about a horizontal axis. In either case, it is necessary that the spindle be raised and lowered relative to the work supporting table.
The spindle heads in machine tools of this type have traditionally been mounted in an external cantilevered relation on the side of the column structure. This arrangement permits the spindle to reach sufficiently far beyond the column to enable reaching of the workpiece during machining, and the base of the spindle assembly in the vertical direction can be chosen sufficiently long to enable it to be captured between relatively long gibs. The long spindle base in the vertical direction is an attempt to provide sufficient rigidity to the spindle assembly to resist tilting about a horizontal axis during machining. Because the spindle assembly is cantilevered, very expensive adjustable gibbing is necessary to give it sufficient rigidity to withstand the side loads encountered during machining. Furthermore, the assembly or arm which supports the spindle is generally constructed of a plurality of parts, which contributes to a reduction in the overall rigidity, and therefore accuracy, of the machine.
One prior art method of very accurately guiding a machine tool arm for vertical movement is to provide precision gibs which are adjusted to define the guidepath for the arm within very close tolerances. An oil film interface between the gibs and arm permits sliding movement, but localized pressures caused by side loads on the spindle arm cause a variation in oil film thickness from gib to gib, thereby affecting close tolerance machining accuracy. Particularly where travel of the arm exceeds several feet, it is extremely difficult to accurately machine and adjust gibs in several axis locations within desired tolerance levels for this type of machining work. The cantilevering of the spindle mounting arm compounds the effect of the side loads, thereby even further affecting machine accuracy. Another factor is thermal growth, which changes the clearance between the gibs and arm.
Not only are the spindle head assemblies of many prior art machine tools of this type quite complex, but also are the mechanisms utilized to translate and rotate the carriage and rotary index table relative to the spindle. In many rotary tables, for example, very complicated bearing arrangements are necessary to ensure that the table can be rotated easily while it supports a heavy workpiece, yet remains substantially level both during and after movement. Additionally, complicated mechanisms are often utilized to accurately locate the table at a desired degree of rotation, and then index it to the subsequent position for the next machining operation. A similar problem exists for accurately moving and locating the carriage at a desired position in front of the spindle supporting column.